Air suspension systems have significant advantages in relation to conventional steel suspension systems and are therefore being increasingly used both in commercial vehicles such as trucks and buses and in primarily heavy passenger cars, such as luxury sedans and SUVs. An air suspension system makes possible ride height control independent of load, since the actual load state can in each case be compensated for by adaptation of the bellows pressure in the spring bellows of the air springs. Likewise, because of the progressive spring characteristics of the air springs, an air suspension system provides especially secure contact of the wheels with the road surface and a comfortable response to bump and rebound movements of the wheels. A further advantage of air suspension systems is that the ground clearance of the vehicles can be varied as required, being increased for off-road use, for example, and reduced for fast driving on motorways. In the case of commercial vehicles, there is the added advantage that the vehicle superstructure can be lowered or set to a suitable height when loading and unloading. For example, the vehicle frame of a pneumatically sprung truck or trailer can be lowered to set down, and raised to receive, an interchangeable platform. Likewise the loading floor of a truck can be adjusted to the level of a loading ramp by reducing or increasing the rear axle bellows pressure to facilitate loading and unloading. In the case of pneumatically sprung buses, the vehicle superstructure on the curbside of the vehicle can be lowered by venting compressed air from the spring bellows on that side to facilitate the entry and egress of passengers and then raised again by recharging the spring bellows.
If the vehicle superstructure is to be raised and lowered only equally at one vehicle axle, a single level-regulating valve is sufficient for charging and venting the spring bellows concerned. In this case, in order to compensate automatically, by means of a small exchange of compressed air, for pressure differences that may occur, for example, through leakage losses on one side, in many air suspension systems a throttled connection between the two or more air spring bellows is provided in the inactive state, that is, with the level-regulating valve closed.
A corresponding valve arrangement is known, for example, from the air suspension system according to DE 195 44 622 C1. In an embodiment of this known air suspension system according to FIG. 5 of that document, the two spring bellows associated with air springs arranged on opposite vehicle sides of a vehicle axle can be charged and vented via a single level-regulating valve and two combined connecting lines. A 3/2-way switching valve by which the two connecting lines are connected to the level-regulating valve in a first switching position (inactive position) and are shut off with respect to the level-regulating valve and connected to one another via a throttle in a second switching position (actuating position) is connected downstream of the level-regulating valve in the venting direction. By means of corresponding activation of an associated pilot valve, the 3/2-way switching valve is held in the actuating position while driving, so that a limited compressed-air or pressure compensation between the two spring bellows can take place. However, one-sided lowering and raising of the vehicle superstructure is not possible with this known air suspension system.
DE 10 2005 032 219 B4 describes embodiments of an air suspension system in which the two spring bellows associated with air springs arranged on opposite vehicle sides of a vehicle axle can each be charged and vented via a respective level-regulating valve arranged in an associated connecting line. The connecting lines of the two spring bellows concerned are connected to one another via a connecting line provided with a throttle and can be shut off by a shut-off valve, so that the limited compressed-air and pressure compensation between the two spring bellows can be interrupted if required. Because the two level-regulating valves can be activated by a common pilot valve, one-sided lowering and raising of the vehicle superstructure is not possible with this known air suspension system.
This disadvantage of the known art can be overcome by separate activation of the two level-regulating valves. This therefore makes it possible to lower and raise the vehicle superstructure on one side, which can be utilized, in the case of a bus for example, to facilitate the entry and egress of passengers. It is also possible in this case, by unequal charging of the spring bellows in the event of unequal load distribution in a truck, to compensate for an oblique stance of the vehicle superstructure that would otherwise occur. For this purpose, however, it is necessary to block the connecting line between the spring bellows by means of the shut-off valve, since, in these cases, pressure compensation between the spring bellows would be disadvantageous and unwanted. The normal configuration of the shut-off valve as a solenoid switching valve or as a pressure-controlled switching valve activatable by a pilot valve in the form of a solenoid switching valve, represents a certain cost factor and is associated with an increased risk of malfunction.